Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of beta-diketiminato group 2 cations, [((BDI)-B-Me)Ae](+) and [(t-BuBDI)Ae](+) ((BDI)-B-Me = HC{(Me)CN-2,6-i-Pr2C6H3}(2); t-BuBDI = HC{(t-Bu)CN-2,6-i-Pr2C6H3}(2); Ae = Mg or Ca), in conjunction with the weakly coordinating anion, [Al{OC(CF3)(3)}(4)](-), allows the characterisation of charge separated alkaline earth eta(6)-pi adducts to toluene or benzene when crystallised from the arene solvents. Addition of 1,4-difluorobenzene to [((BDI)-B-Me)Mg](+) results in the isolation of [((BDI)-B-Me)Mg(1,4-F2C6H4)(3)](+) in which the fluorobenzene molecules coordinate via kappa(1)-F-M interactions. Although DFT analysis indicates that the polyhapto arene binding to Mg is effectively electrostatic in origin, the interactions with Ca (Sr and Ba) are observed to invoke small but significant pi overlap of the arene HOMOs with the alkaline earth valence nd orbitals. Reaction of triphenylphosphine with [((BDI)-B-Me)Mg](+) and [(t-BuBDI)Mg](+) in toluene solvent allows the isolation of the respective terminally coordinated magnesium-phosphine adducts. The resultant Mg-P bond lengths [2.5972(13), 2.6805(12) angstrom] are comparable to those previously observed in magnesium derivatives of terminal but formally anionic phosphide ligands, while the effectively electrostatic nature of the bonding is supported by DFT calculations.